Air breathing turbines typically include, as major components, a turbine wheel coupled to a rotary compressor. A combustor receives compressed air from the compressor as well as fuel from a fuel source and burns the same to provide hot gasses of combustion to drive the turbine wheel.
Many such systems employ so-called "annular" combustors. Such a combustor includes a somewhat toroidal-shaped combustion chamber centered about the rotational axis of the turbine wheel and there are provided a plurality of circumferentially or angularly spaced fuel injectors which inject fuel into the annular combustion space as well as air to support combustion during normal operation.
In the usual case, the injectors are of two different types. One type is a so-called "start" injector and the other type is a so-called "main" injector. Generally speaking, the main injectors will substantially outnumber the start injectors and the start injectors are employed only during the start-up sequence for the turbine. The main injectors are not used to initiate start-up of the turbine but may be employed in later stages of a start-up operation as well as during normal turbine operation.
Generally speaking, the start injectors are configured to provide for good atomization of the fuel. Good atomization at the injection nozzles is necessary because during start-up, the velocity of compressed air received from the compressor is relatively low (because the apparatus is rotating at a relatively low speed). Thus, air velocity cannot be utilized to enhance atomization.
Frequently, to achieve the enhanced atomization of fuel that is required at low engine rotational speeds, relatively high volumes of fuel are pumped through those nozzles, nozzles typically providing better atomization for higher pressure drops across the nozzle and/or higher flow rates. This, however, results in local overfueling which in turn causes combustion inefficiencies and damaging hot streaks, i.e., the formation of undesirable hot spots which may damage the combustor. Thus, conventional starting systems have drawbacks in terms of requiring two different types of injectors (which tends to multiply the total number of injectors involved and thereby increase expense), requiring a relatively large number of control valves, and being prone to the formation of undesirable hot spots.
The present invention is directed to overcoming one or more of the above problems.